Embodiments of the invention relate generally to alternating current (AC) motors and, more particularly, to a system and method for measuring current flowing through a motor soft starter.
Motor soft starters are devices that control transmission of voltage and current from an AC power source to an induction motor. Soft starters are configured to limit the transient voltages and current to the induction motor during start-up, resulting in a “soft” motor starting. In operation, power from the AC source is passed through switching devices in the soft starter, such as a pair of anti-parallel solid-state switches in the form of thyristors or silicon controlled rectifiers (SCRs), to control the current flow and, in turn, the terminal voltages of the induction motor.
In general, the soft starter temporarily reduces load and torque in a powertrain of the motor during startup. This reduction allows for reduced stresses on the motor and electrical network, which increases the life of the system. The soft starter or motor drive allows for reducing the voltage or current input to the motor via selective control of the thyristors. A failure of one or more thyristors in the soft starter may lead to system inoperability or to elimination of the soft starting technique for extending the life of the motor.
Detection devices have been designed that generate feedback regarding the amount of current flowing through the soft starter. The feedback may be monitored to determine the power dissipation through the soft starter and may be used to calculate the temperature of the soft starter for heat regulation. A common industrial practice is to measure current using the same principles as a transformer. A magnetic field is induced around a conductor as current is passed through the conductor. This magnetic field may be induced into a magnetic coil looped around the conductor. This method is similar to an air core transformer and is commonly referred to as a current transformer. The amount of magnetically induced current into the coil is dependent on the number of coil loops and the amount of signal current desired. The current signal, therefore, should be proportional to the actual current in the conductor of interest. A scale is developed to read the coupled current signal value in the conductor as an actual current signal.
The output of the current transformer may be used to sense an on-state condition in the soft starter by sensing a high current passing through the conductor, for example. Sensing a high current through the soft starter includes sensing a wide range of current that may pass through the conductor. Often, the sensed current is converted from an analog signal to a digital signal. Sensing and converting such a wide range of current to a digital signal results in a large step size between each digital value. Such a coarse digital step size scale is often adequate to estimate the power flowing through the soft starter. However, it may be desirable to sense the currents flowing through thyristors of the soft starter both in the on state as well as in the off state to more accurately measure the operational status of the soft starter. In this case, the coarse digital step size of the scale set up for wide-range current detection is often too large to provide reliable accurate information for currents flowing through off-state thyristors.
It would therefore be desirable to have a system for more accurately sensing the current flowing through a soft starter.